Comprehensive and Detailed In-Depth Explanation:
System software controls are mechanisms designed to protect system integrity, security, and performance.
Among the given options, performing intrusion testing on a regular basis (D) is a proactive security measure that tests an organization's IT infrastructure to identify vulnerabilities and weaknesses in system security.
Option A (Restricting server room access) is a physical security control, not a system software control.
Option B (Housing servers securely) is an environmental control, focusing on protecting hardware.
Option C (Ensuring documentation of user requirements) relates to project management and system development, rather than system software security.
Since intrusion testing ensures system resilience against cyber threats, option D is the correct answer.
Reference: IIA CIA Exam Syllabus - IT Security & Controls